1. Field of the Invention
The present invention relates to an arrangement for causing, from a telephone set, opening of the subscriber loop for a well-defined time interval in response to the actuation of a special key of the set which renders a predominantly capacitive control link operative. This arrangement comprises inter alia a diode bridge which ensures a fixed polarity of end portions of the line wires downstream of the hook, at least one transmission and dialling circuit, and a monostable pulse circuit for opening the loop. The monostable circuit comprises at least a first transistor of the bipolar type which controls a second transistor which is normally conductive and whose main current path is arranged in series with at least one of the transmission and dialling circuits, provided at the end of the end portions of the line wires for normal closure of the loop. The time interval during which the loop is open is determined by the time constant produced by a timing resistor arranged in series with a timing capacitor which is charged to a voltage defined by a Zener diode, whilst the end of the loop opening interval is defined by an end-of-charging voltage which is less than the zener voltage.
2. Description of the Prior Art
Such an arrangement is disclosed in French Patent Application No. 2471704.
It is known that novel electronic automatic change-over devices provide additional services to the telephone subscribers, such as the indication that a call is being established, a conference with three participants, call-back facility, etc. In order to get access to these services, it is necessary that one can return from the communication phase to the dialling phase, which operation is controlled from the subscriber set and requires a specific signal to be transmitted over the line. This signal consists in opening the loop for a predetermined interval during given limits, for example: 270 ms, plus or minus 50 ms.
It should be noted that a certain number of complementary requirements must also be satisfied during the operation of such an arrangement, so that the feasible technical solutions are not as simple as might be expected at a first glance.
It is indeed necessary to remain within prescribed limits for the duration of the pulse whatever the manner in which the relevant key has been actuated. On the other hand, during the interval in which the loop is open, the residual leakage current on the line must be very low, for example less than 200 .mu.A, and this even when the line supply voltage is raised to approximately 100 V (in the case of lines with enhanced supply). Finally, outside the loop-opening period, the arrangement must not consume appreciable energy and must not disturb in any way the operation of the set, neither during the communication phase nor during the dialling phase. It is, more specifically, necessary that during the communication phase the characteristic impedance with which the line is loaded is not disturbed and also that at the beginning of the dialling phase, when voltage is applied to the circuit downstream of the hook, the arrangement does not introduce any additional delay for the current flow through the standardized line in the off-hook condition of the set.